Make sure you don't go the route of building out of m2's like i did, you will have issues with collision.
It's possibly a good idea to build something in noggit out of models you can find which match your ideas, but never build a final product with them not for wall structure at least. Amaroth made a video about this it's on his channel, however it was way after i had finished making this ( Ended up being a waste of time)
Build the model with a 3D application such as blender/3ds max/maya
Here's the video explaining why it shouldn't be done

Client
Make sure that MAX_RACES in charactercreate.lua is set to the correct value IE 12 for your case.
Server
Update MAX_RACES in shareddefines.h to +1 of the required value for two more custom races it would be set to 13 not 12.